1. Field of the Invention
This invention relates generally to lasers, and more specifically to diode-pumped solid-state blue microlasers which employ second harmonic generation.
2. Discussion of the Related Art
Many developments have been made concerning the generation of harmonic radiation from solid-state laser devices. These advances relate both to the efficiency with which such devices operate and to the output wavelengths which have been obtained. For applications requiring small-to-modest levels of optical power, an attractive form of solid-state laser has been that of the microlaser. Such a device comprises a monolithic or composite resonant cavity wherein a diode laser is used to pump a short element of solid-state gain medium, the latter being formed by reflective surfaces on opposite ends of the cavity.
The solid-state gain medium consists of a rare-earth ion, such as trivalent neodymium (Nd.sup.3+), doped into a suitable host material. The most well studied gain medium is Nd.sup.3+ -doped yttrium aluminum garnet (Nd:YAG) which has been diode laser-pumped and has been made to lase at either 1342 nm, 1064 nm or 946 nm.
In Zayhowski, "Microchip Lasers," The Lincoln Laboratory Journal, Vol. 3, No. 3, pp. 427-445 (1990), the demonstration of single-frequency microchip lasers which use a miniature, monolithic, flat-flat, solid-state cavity whose longitudinal mode spacing is greater than the gain bandwidth of the gain medium, is reported. These microchip lasers are longitudinally pumped with the close-coupled, unfocused output of a laser diode to generate near-infrared radiation. Mooradian has disclosed in U.S. Pat. No. 4,860,304 a microlaser employing a gain medium made from Nd:YAG having a cavity length that is less than 700 .mu.m.
Nonlinear optical crystals can be used to convert near-infrared radiation to the visible portion of the spectrum via second harmonic generation (SHG) (sometimes termed frequency doubling). This process generates a harmonic wavelength which is one-half of the fundamental wavelength. Since the SHG conversion efficiency is a function of the fundamental laser beam intensity, the nonlinear crystal is often placed inside the cavity of a low power continuous wave laser to benefit from the high intracavity fundamental beam intensity. This technique is well known and is discussed by Mooradian in U.S. Pat. No. 4,953,166 where a solid-state gain material is bonded to a SHG nonlinear crystal. Dielectric reflective coatings are deposited directly to the gain and nonlinear material surfaces to form a composite cavity intracavity doubled laser. However, the main intent of this reference is to teach configurations which give rise to single frequency operation by selecting the cavity length such that the gain bandwidth is less than or substantially equal to the frequency separation of the cavity modes. This is not the case with the present disclosure.
A typical SHG method is intracavity doubling using KTP (potassium-titanyl-phosphate, or KTiOPO.sub.4) as a nonlinear crystal. Radiation at 532-nm with as much as a few watts of power has been obtained in this way with a longitudinally oriented, diode laser pumped Nd:YAG laser. Additionally, 4 mW at 473 nm has been achieved using KNbO.sub.3 (potassium niobate) as an intracavity SHG crystal in combination with Nd:YAG lasing at 946 nm [Risk et al., Appl. Phys. Lett. 54 (17), 1625 (1989)]. However, diode laser pumped blue-green lasers based on intracavity SHG of Nd:YAG are limited in efficiency and stability due to the unpolarized emission, and the relatively weak, narrow diode absorption features of Nd:YAG. The conversion of optical radiation at one frequency into optical radiation of another frequency by integration with a nonlinear optical material within an optical cavity is known. Byer et al., in U.S. Pat. Nos. 4,739,507 and 4,731,787, disclose a diode-pumped laser having a harmonic generator. In U.S. Pat. No. 4,809,291, Byer describes a diode-pumped solid-state laser which is frequency doubled to produce blue light. Byer also discusses the same subject in the article "Diode Laser-Pumped Solid-State Lasers" Science, Vol. 239, p. 745 (1988). In these documents there is no mention of a requirement for polarized emission and/or broad absorption lines.
In contrast to the above mentioned blue-green solid-state lasers based on Nd:YAG, the uniaxial crystal Nd:YVO.sub.4 (neodymium-doped yttrium orthovanadate) has polarized emission and strong, wide absorption transitions [Yaney et al., J. Opt. Soc. Am., 66, 1405 (1976)]. Diode laser pumped operation of Nd:YVO.sub.4 lasing at 1064 nm with intracavity SHG to 532 nm utilizing KTP has been demonstrated to be more stable and efficient than Nd:YAG based systems [see Sasaki et al., Optics Letters, Vol, 16(21), 1665 (1991) and Tatsumo et al., "Highly Efficient and Stable Green Microlaser Consisting of Nd:YVO.sub.4 with Intracavity KTP for Optical Storage," paper CWQS, CLEO-92, Anaheim, Calif. (May 1992)].
Kintz et al. in U.S. Pat. No. 4,942,582 disclose a technique for generating single frequency output from Nd:YVO.sub.4 based lasers. Additionally, intracavity SHG utilizing KTP is claimed but no specific wavelengths or transitions are discussed. This patent specifically teaches the use of external, separate mirrors for the output coupler which does not correlate with the currently described invention.
Consequently, although in separate references the concept of intracavity doubled microlasers and of blue light production are recognized as practical, nowhere is there a teaching relative to blue microlasers which employ the .sup.4 F.sub.3/2 .fwdarw..sup.4 I.sub.9/2 914-nm emission line of Nd:YVO.sub.4 to produce blue radiation at 457 nm via second harmonic generation in BBO or KNbO.sub.3. Nor is there any recognition that low lasant ion concentrations, on the order of less than 2.0%, permit the efficient production of blue light to occur. Additionally, there is no requirement or suggestion in the known prior art for either polarized emission or broad absorption bands of the laser material.